﻿using System;
using System.Text;
namespace Helpers
{
/// <summary>
/// 提供用于计算指定文件哈希值的方法
/// <example>例如计算文件的MD5值:
/// <code>
///   String hashMd5=HashHelper.ComputeMD5("MyFile.txt");
/// </code>
/// </example>
/// <example>例如计算文件的CRC32值:
/// <code>
///   String hashCrc32 = HashHelper.ComputeCRC32("MyFile.txt");
/// </code>
/// </example>
/// <example>例如计算文件的SHA1值:
/// <code>
///   String hashSha1 =HashHelper.ComputeSHA1("MyFile.txt");
/// </code>
/// </example>
/// </summary>
public sealed class HashHelper
{
    /// <summary>
    ///  计算指定文件的MD5值
    /// </summary>
    /// <param name="fileName">指定文件的完全限定名称</param>
    /// <returns>返回值的字符串形式</returns>
    public static String ComputeMD5(String fileName)
    {
        String hashMD5 = String.Empty;
        //检查文件是否存在，如果文件存在则进行计算，否则返回空值
        if (System.IO.File.Exists(fileName))
        {
            using (System.IO.FileStream fs = new System.IO.FileStream(fileName, System.IO.FileMode.Open, System.IO.FileAccess.Read))
            {
                //计算文件的MD5值
                System.Security.Cryptography.MD5 calculator = System.Security.Cryptography.MD5.Create();
                Byte[] buffer = calculator.ComputeHash(fs);
                calculator.Clear();
                //将字节数组转换成十六进制的字符串形式
                StringBuilder stringBuilder = new StringBuilder();
                for (int i = 0; i < buffer.Length; i++)
                {
                    stringBuilder.Append(buffer[i].ToString("x2"));
                }
                hashMD5 = stringBuilder.ToString();
            }//关闭文件流
        }//结束计算
        return hashMD5;
    }//ComputeMD5
     /// <summary>
     ///  计算指定文件的CRC32值
     /// </summary>
     /// <param name="fileName">指定文件的完全限定名称</param>
     /// <returns>返回值的字符串形式</returns>
    public static String ComputeCRC32(String fileName)
    {
        String hashCRC32 = String.Empty;
        //检查文件是否存在，如果文件存在则进行计算，否则返回空值
        if (System.IO.File.Exists(fileName))
        {
            using (System.IO.FileStream fs = new System.IO.FileStream(fileName, System.IO.FileMode.Open, System.IO.FileAccess.Read))
            {
                //计算文件的CSC32值
                Crc32 calculator = new Crc32();
                Byte[] buffer = calculator.ComputeHash(fs);
                calculator.Clear();
                //将字节数组转换成十六进制的字符串形式
                StringBuilder stringBuilder = new StringBuilder();
                for (int i = 0; i < buffer.Length; i++)
                {
                    stringBuilder.Append(buffer[i].ToString("x2"));
                }
                hashCRC32 = stringBuilder.ToString();
            }//关闭文件流
        }
        return hashCRC32;
    }//ComputeCRC32
     /// <summary>
     ///  计算指定文件的SHA1值
     /// </summary>
     /// <param name="fileName">指定文件的完全限定名称</param>
     /// <returns>返回值的字符串形式</returns>
    public static String ComputeSHA1(String fileName)
    {
        String hashSHA1 = String.Empty;
        //检查文件是否存在，如果文件存在则进行计算，否则返回空值
        if (System.IO.File.Exists(fileName))
        {
            using (System.IO.FileStream fs = new System.IO.FileStream(fileName, System.IO.FileMode.Open, System.IO.FileAccess.Read))
            {
                //计算文件的SHA1值
                System.Security.Cryptography.SHA1 calculator = System.Security.Cryptography.SHA1.Create();
                Byte[] buffer = calculator.ComputeHash(fs);
                calculator.Clear();
                //将字节数组转换成十六进制的字符串形式
                StringBuilder stringBuilder = new StringBuilder();
                for (int i = 0; i < buffer.Length; i++)
                {
                    stringBuilder.Append(buffer[i].ToString("x2"));
                }
                hashSHA1 = stringBuilder.ToString();
            }//关闭文件流
        }
        return hashSHA1;
    }//ComputeSHA1
}//end class: HashHelper
 /// <summary>
 /// 提供 CRC32 算法的实现
 /// </summary>
public class Crc32 : System.Security.Cryptography.HashAlgorithm
{
    public const UInt32 DefaultPolynomial = 0xedb88320;
    public const UInt32 DefaultSeed = 0xffffffff;
    private UInt32 hash;
    private UInt32 seed;
    private UInt32[] table;
    private static UInt32[] defaultTable;
    public Crc32()
    {
        table = InitializeTable(DefaultPolynomial);
        seed = DefaultSeed;
        Initialize();
    }
    public Crc32(UInt32 polynomial, UInt32 seed)
    {
        table = InitializeTable(polynomial);
        this.seed = seed;
        Initialize();
    }
    public override void Initialize()
    {
        hash = seed;
    }
    protected override void HashCore(byte[] buffer, int start, int length)
    {
        hash = CalculateHash(table, hash, buffer, start, length);
    }
    protected override byte[] HashFinal()
    {
        byte[] hashBuffer = UInt32ToBigEndianBytes(~hash);
        this.HashValue = hashBuffer;
        return hashBuffer;
    }
    public static UInt32 Compute(byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(DefaultPolynomial), DefaultSeed, buffer, 0, buffer.Length);
    }
    public static UInt32 Compute(UInt32 seed, byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(DefaultPolynomial), seed, buffer, 0, buffer.Length);
    }
    public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
    }
    private static UInt32[] InitializeTable(UInt32 polynomial)
    {
        if (polynomial == DefaultPolynomial && defaultTable != null)
        {
            return defaultTable;
        }
        UInt32[] createTable = new UInt32[256];
        for (int i = 0; i < 256; i++)
        {
            UInt32 entry = (UInt32)i;
            for (int j = 0; j < 8; j++)
            {
                if ((entry & 1) == 1)
                    entry = (entry >> 1) ^ polynomial;
                else
                    entry = entry >> 1;
            }
            createTable[i] = entry;
        }
        if (polynomial == DefaultPolynomial)
        {
            defaultTable = createTable;
        }
        return createTable;
    }
    private static UInt32 CalculateHash(UInt32[] table, UInt32 seed, byte[] buffer, int start, int size)
    {
        UInt32 crc = seed;
        for (int i = start; i < size; i++)
        {
            unchecked
            {
                crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
            }
        }
        return crc;
    }
    private byte[] UInt32ToBigEndianBytes(UInt32 x)
    {
        return new byte[] { (byte)((x >> 24) & 0xff), (byte)((x >> 16) & 0xff), (byte)((x >> 8) & 0xff), (byte)(x & 0xff) };
    }
}//end class: Crc32


}